1. Field of the Invention
The present invention relates to a frictional dynamic characteristic measuring apparatus for measuring a pressing force and a frictional force acting between a friction material as a specimen and a slide portion when the friction material and the slide portion are in sliding contact. The frictional dynamic characteristic measuring apparatus of the present invention comprises a sliding speed changing means for self-excitedly changing a sliding speed of the friction material and the sliding portion at a resonance frequency of a resonance system including the friction material and the slide portion, and for forcibly changing the sliding speed of the friction material and the sliding portion at the resonance frequency of the resonance system, thereby enabling evaluation of the possibility of occurrence of vibration of the friction material that absorbs self-excited vibration of a torsional vibration system.
2. Description of the Related Art
Frictional vibration sometimes occurs in an apparatus for driving or braking using a friction material. If the velocity gradient of the friction coefficient of the friction material during the frictional vibration can be measured by a measuring apparatus under the same conditions as for actual frictional vibration in the aforementioned apparatus, it becomes possible to take countermeasures against frictional vibration in the aforementioned apparatus considering characteristics of only the friction material, thereby simplifying tests in the aforementioned apparatus or the like.
Since the aforementioned frictional vibration is greatly dependent on the velocity gradient of friction coefficient (d.mu./dV value), the characteristic is used as an evaluation item. If the velocity gradient of friction coefficient is negative, frictional force, that is, the product of a load that perpendicularly acts on a friction surface of the friction material and a friction coefficient of a vibrating body on the friction surface, acts to maintain the vibration or to increase the vibration amplitude, corresponding to a speed change caused by vibration.
Conventionally, to measure the velocity gradient of a friction coefficient for evaluation of a friction material alone, a change in the friction coefficient relative to an objective velocity range or a specific velocity range is determined by selecting several velocities within such a range and measuring a friction coefficient at each velocity selected, i.e. at a constant velocity. However, this method does not determine an appropriate velocity gradient since the velocity changes in accordance with oscillation frequency under actual vibrating conditions. Therefore, the conventional method for measuring friction characteristic of a friction material alone cannot predict an actual phenomenon, but requires further evaluation in an actual use for final verification, in addition to the material evaluation, thus requiring considerable time and labor.
In conventional test apparatuses (Japanese Patent Laid-Open No. Sho 62-832, Japanese Patent Laid-Open No. Hei 05-126683) for simulating vibrations that occur in actual apparatuses, a vibration system similar to that in an actual apparatus is set up and realized in a test apparatus, and vibrations expected to occur in the actual apparatus are simulated. Depending on whether vibration occurs in the test apparatus, occurrence of vibrations in the actual apparatus is predicted.
The aforementioned conventional vibration simulating test apparatus simulates vibrations expected to occur in an actual apparatus, and makes it possible to predict occurrence of vibrations in the actual apparatus depending on whether vibration occurs in the test apparatus. There is a problem, however, that what can be evaluated by this apparatus is limited to materials capable of causing frictional vibrations equal to or greater than a certain extent.
More specifically, the conventional vibration simulating test apparatus has a problem that it merely evaluates whether the frictional material is a material that allows friction vibrations.
FIGS. 5 and 6 indicate results of measurement of materials with different evaluations as for frictional vibration. Material A in FIG. 5 is a material that readily undergoes frictional vibration, so that material A vibrates at an eigenfrequency of the friction characteristic measuring apparatus. On the other hand, material B in FIG. 6 is a material that does not readily undergo frictional vibration, that is, a material that has a characteristic of absorbing self-excited vibration in a torsional vibration system, so that no vibration occurs. This evaluation apparatus can evaluate materials in terms of whether vibration occurs, if the characteristics of materials considerably differ as in material A and material B. However, if a third material C undergoes no vibration as in material B, the superiority between material C and material B cannot be determined. Thus, this evaluation apparatus has a problem of incapability to evaluate the possibility of occurrence of vibration in a friction material having a characteristic of absorbing self-excited vibration in a torsional vibration system.